1. Field of the Invention
The present invention relates to a heat sensitive flow amount sensor for measuring the amount of intake air flowing, for example, in the inlet pipe of an internal combustion engine and an inlet system of an internal combustion engine.
2. Description of the Related Art
FIG. 16 is a side sectional view of the main portion of an inlet system of an internal combustion engine disclosed in Japanese Patent Application Laid-open No. 64-36937. FIG. 16, shows an inlet pipe 1 with a throttle valve 2 disposed therein which turns in accordance with the amount of depression of an accelerator pedal to thereby regulate the air flow amount to be taken into the cylinders (not shown). A bypass 3 is disposed upstream of the throttle valve 2 and when the internal combustion engine is in an idling state or the like and the throttle valve 2 is totally closed, intake air is introduced into the cylinders through the bypass 3. Further, a heat sensitive flow amount sensor 4 is disposed upstream of the throttle valve 2 in the inlet pipe 1 to measure the an intake air flow amount in the inlet pipe 1.
The heat sensitive flow amount sensor 4 includes: a cylindrical measuring pipe 5, a column 6 for supporting the measuring pipe 5, an air flow amount sensing element 7 which is disposed in the measuring pipe 5 and heated with its resistance value changing according to the temperature, a heat sensitive resistive element (not shown) which is disposed in the inlet pipe 1 whose resistance value changes according to the temperature to thereby detect the temperature of intake air, and a control circuit 9 coupled with the column 6. The control circuit 9 is electrically connected to the air flow amount sensing element 7 and the heat sensitive resistive element, and heats the air flow amount sensing element 7 so that it always maintains a certain temperature difference with respect to the temperature of the intake air in the inlet pipe 1 detected by said heat sensitive resistive element, as well as converts a current corresponding to the heating thereof into an output voltage signal.
In the inlet system of the internal combustion engine arranged as described above, a portion of the intake air which has passed through an air cleaner (not shown) and flowed into the inlet pipe 1 flows into the measuring pipe 5. At that time, although heat is taken from the air flow amount sensing element 7 in accordance with the flow amount of the intake air, the air flow amount sensing element 7 is subjected to constant temperature difference control by the control circuit 9, so that it always maintains the certain temperature difference with respect to the temperature of the intake air detected by the heat sensitive resistive element. Since the control circuit 9 converts the current corresponding to the heating of the air flow amount sensing element 7 into the output voltage signal corresponding to the air flow amount, it can determine the amount of the intake air in the inlet pipe 1 from the output voltage signal. In this way, the amount of air flowing in the inlet pipe 1 is measured in the measuring pipe 5 of the heat sensitive flow amount sensor 4. The air flow in the inlet pipe 1 is affected by the degree which the throttle valve 2 is opened, and by the bypass 3, etc, each of which disturbs the air flowing into the measuring pipe 5 and reduces the accuracy of the measurement of the air flow. Therefore, the heat sensitive flow amount sensor 4 must be installed as far away from the throttle valve 2 as possible to reduce the effect of the above factors as much as possible, and thus accurately measure the flow amount of intake air.
The conventional inlet system of an internal combustion engine has the following problems: since the heat sensitive flow amount sensor 4 must be installed as far way from the throttle valve 2 as possible to accurately measure the flow amount of intake air, the inlet system of the internal combustion engine is increased in size and, the space where the internal combustion engine is mounted increases as does the manufacturing cost.
An object of the present invention is to solve the above problem and to provide a heat sensitive flow amount sensor which can measure an air flow amount without lowering an measuring accuracy even if the sensor is installed in the vicinity of, for example, a throttle valve, and contribute to the reduction in size of an internal combustion engine, and acheive a cost reduction for a heat sensitive flow amount senosr and the inlet system of an internal combustion engine.